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Olivine-Dominated Asteroids: Mineralogy and Origin

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Olivine-Dominated Asteroids: Mineralogy and Origin Olivine-dominated asteroids: Mineralogy and origin Juan A. Sancheza,1,∗, Vishnu Reddyb,1, Michael S. Kelleyc,1,2, Edward A. Cloutisd, William F. Bottkee, David Nesvorn´ye, Michael P. Lucasf, Paul S. Harderseng,1, Michael J. Gaffeyg,1, Paul A. Abellh,1, Lucille Le Correb aMax Planck Institut fu¨r Sonnensystemforschung, Katlenburg-Lindau, Germany bPlanetary Science Institute, 1700 East Fort Lowell Road, Tucson, Arizona 85719, USA cDepartment of Geology and Geography, Georgia Southern University, Statesboro, USA dDepartment of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada eSouthwest Research Institute and NASA Lunar Science Institute, Boulder, USA fDepartment of Earth and Planetary Sciences, University of Tennessee, USA gDepartment of Space Studies, University of North Dakota, Grand Forks, USA hNASA Johnson Space Center, Houston, Texas, USA Abstract Olivine-dominated asteroids are a rare type of objects formed either in nebular processes or through magmatic differentiation. The analysis of meteorite samples suggest that at least 100 parent bodies in the main belt experienced partial or complete melting and differentiation before being disrupted. However, only a few olivine-dominated asteroids, representative of the mantle of disrupted differentiated bodies, are known to exist. Due to the paucity of these ob- jects in the main belt their origin and evolution have been a matter of great debate over the years. In this work we present a detailed mineralogical analysis of twelve olivine-dominated asteroids. We have obtained near-infrared (NIR) spectra (0.7 to 2.4 µm) of asteroids (246) Asporina, (289) Nenetta, (446) Aeternitas, (863) Benkoela, (4125) Lew Allen and (4490) Bam- berry. Observations were conducted with the Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i. This sample was complemented with spectra of six other olivine-dominated asteroids including (354) Eleonora, (984) Gretia, (1951) Lick, (2501) Lohja, (3819) Robinson and (5261) Eureka obtained by previous workers. Within our sample we distinguish two classes, one that we call monomineralic-olivine asteroids, which are those whose spectra only exhibit the 1 µm feature, and another referred to as olivine-rich asteroids, whose spectra exhibit the 1 µm feature and a weak (Band II depth ∼ 4%) 2 µm feature. For the monomineralic-olivine asteroids the olivine chemistry was found to range from ∼ Fo49 to Fo70, consistent with the values measured for brachinites and R chondrites. In the case of the olivine-rich asteroids we determined their olivine and low-Ca pyroxene abundance using a new set of spectral calibrations derived from the analysis of R chondrites spectra. We found that the olivine abundance for these asteroids varies from 0.68 to 0.93, while the fraction of low-Ca pyroxene to total pyroxene ranges from 0.6 to 0.9. A search for dynamical connections between the olivine-dominated asteroids and arXiv:1310.1080v2 [astro-ph.EP] 12 Nov 2013 asteroid families found no genetic link (of the type core-mantel-crust) between these objects. Keywords: ∗ Corresponding author at: Max Planck Institut f¨ur Sonnensystemforschung, Max Planck Str.2, 37191 Katlenburg-Lindau, Germany. Email address: [email protected] (Juan A. Sanchez) 1Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement No. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. 2Planetary Science Division, Science Mission Directorate, NASA Headquarters, Washington, DC 20546, USA. Published in Icarus October 8, 2018 Asteroids, Spectroscopy, Infrared observations, Meteorites 1. Introduction A-type asteroids are a unique class of objects that were initially distinguished from the R-type asteroids (the group into which they’d previously been classified) based on broadband spectrophotometry by Veeder et al. (1983) and were later re-classified based on Eight Color Asteroid Survey (ECAS) data (0.3 to 1.1 µm) by Tholen (1984). Asteroids of this taxonomic class have moderately high albedos, extremely reddish slopes shortward of 0.7 µm, and a strong absorption feature centered at ∼1.05 µm (Tholen and Barucci, 1989). Subsequent near-infrared (NIR) spectra have shown that in these original A-type asteroids a ∼2 µm feature is absent or very weak, consistent with a silicate component of nearly monomineralic olivine on the surface of these bodies. The discovery of olivine-dominated asteroids is of considerable interest regarding the ac- cretion and geochemical evolution of primitive bodies. Olivine-dominated objects are expected to form either through magmatic differentiation, being the major constituent of the mantles of most differentiated bodies (Burbine et al., 1996), or through nebular processes which can produce olivine-dominated objects like the R-chondrite parent body (Schulze et al., 1994). The presence of olivine-dominated asteroids suggests that at least some objects in the asteroid belt underwent complete or near-complete melting that led to the differentiation of their interiors. Another interesting aspect is that in order for the mantle to be exposed, the parent body must be fragmented or its deep interior exposed by large impacts. Based on meteorites in terrestrial collections, it is estimated that at least ∼ 100 meteorite parent bodies should have existed in the asteroid belt that underwent partial or complete melting and differentiation before disrup- tion and fragmentation (Keil, 2000). However, even assuming all A-type asteroids are olivine- dominated, only a handful of objects from the mantles of differentiated and disrupted parent bodies were discovered during the taxonomic surveys. This is described as the ”missing mantle” problem because the corresponding mantle components of the iron cores (as represented by iron meteorites) are missing (Chapman, 1986; Bell et al., 1989; Burbine et al., 1996). More recent work on M-type asteroids by Hardersen et al. (2011) indicates that a subset of that population (766 Moguntia, 798 Ruth and 1210 Morosovia) shows a significant olivine component in the surface assemblage. It is unclear, at this point, if the olivine seen on these M-type asteroids are pieces of mantle remaining on an iron-rich core or formed via nebular processes. More recent surveys like the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) (Xu et al., 1995), and SMASS II (Bus and Binzel, 2002a,b) have expanded the number of members within each taxonomic class based on visible spectroscopy. Twelve new A-type as- teroids were added to the original five from Tholen (1984). Burbine and Binzel (2002) ob- served 10 A-type asteroids at near-infrared wavelengths, four from Tholen (1984) and six from Bus and Binzel (2002a,b). They subsequently divided the A-type asteroids into two groups based on the strength of the 1 µm feature. Because the Burbine and Binzel (2002) data do not extend beyond 1.65 µm, the possibility of a 2 µm feature due to pyroxene cannot be ruled out. It is important to note that A-type asteroids under the Bus and Binzel (2002a,b) taxo- nomic system are not the same as those under the original Tholen system. Some A-types in the SMASS II taxonomic system contain up to ∼ 20% pyroxene, as indicated by the presence of a 2 µm feature in NIR data, e.g., (4142) Dersu-Uzala (Binzel et al., 2004), and are similar to S-I/S-II asteroids in the Gaffey S-asteroid subtypes (Gaffey et al., 1993). A comprehensive summary of all previous work on A-type asteroids is published in Sun- shine et al. (2007). This study was based on the work of Sunshine and Pieters (1998), and included VIS-NIR spectra of nine olivine-dominated asteroids. Of these nine objects, four were 2 analyzed using the Modified Gaussian Model (MGM) (Sunshine et al., 1990) in order to derive their olivine compositions. Those four objects that included (1951) Lick, (289) Nenetta, (246) Asporina, and (354) Eleonora were characterized by the lack of a detectable 2 µm feature. The other five asteroids; (446) Aeternitas, (863) Benkoela, (984) Gretia, (2501) Lohja, and (3819) Robinson, whose spectra have a detectable 2 µm feature, were not analyzed due to the difficulties inherent to the modeling of olivine-pyroxene mixtures (Sunshine et al., 2007). In the present work we analyze VIS-NIR spectra of twelve olivine-dominated asteroids, six observed by our group and six obtained from previous studies. Because taxonomic classification can be ambiguous depending on the system used we will refer to these objects as S(I)-types, which is the designation introduced by Gaffey et al. (1993) that includes objects where olivine is the major silicate phase present. We further distinguish two classes within our sample: one class that will be called monomineralic-olivine asteroids, which are those whose spectra exhibit the 1 µm feature and no detectable 2 µm feature, and another class that will be called olivine-rich asteroids, whose spectra exhibit the 1 µm feature and a weak 2 µm feature. The approach used in the present study differs from previous work (e.g., Sunshine and Pieters, 1998; Sunshine et al., 2007) in that olivine compositions are determined from the measured Band I centers, along with a spectral calibration derived from laboratory measurements. Furthermore, in the case of the olivine-rich asteroids, the olivine-pyroxene abundance ratio (ol/(ol+px)) and the ratio of low-Ca pyroxene (LCP) to total pyroxene (LCP/(LCP+HCP)) are determined us- ing a set of equations derived from the analysis of meteorite samples. Here we define low-Ca pyroxenes (LCP) as pyroxenes with <25% iron and include pigeonite and orthopyroxene, and high-Ca pyroxenes (HCP) as those with >25% iron, including augite-diopside-hedenbergite. In addition to the mineralogical analysis we also search for dynamical connections between the studied objects and asteroid families. Using this information we finally discuss possible formation scenarios for the olivine-dominated asteroids. 2.
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